The present invention relates to data migration in snapshot operations in a data storage system.
A data storage system may employ snapshots (i.e., an image of data at an instant of time) for a variety of operations including backup, data mining, or testing of software programs. A snapshot operation results in two data images: (1) the source data also referred to as active data that can be read and written by applications as soon as the snapshot is created, and (2) the snapshot data that can be backed up or mined or otherwise used. The need for high data availability often requires frequent snapshots that consume resources such as cache memory, internal memory bandwidth, storage device capacity and the storage device bandwidth.
Data storage systems use several techniques to migrate the snapshot data from the source VLUN to the target VLUN so that the source data can be further modified without loss of the snapshot data. Some of these techniques include (1) a complete copy snapshot, and (2) a partial copy snapshot with separate VLUNs. In a complete copy snapshot operation, the data storage system allocates a new target VLUN for each snapshot that equals the amount of storage provided by the source VLUN at the time of the snapshot. If metadata is also stored with the snapshot data, the space of the target VLUN will be greater than that of the source VLUN. Each target VLUN will store a complete copy of the snapshot data upon migration of the original data from the source VLUN. This technique is simple, but consumes considerable storage capacity especially if there are frequent snapshots. A partial copy snapshot operation still uses multiple target VLUNs to store the snapshot data, but may allocate less storage space since the original data is only migrated when it is modified.